The Hatchlings appear in the episode "The Witch's Garden." They started hatching from 3 dark-green colored eggs as soon after Finn was brought to the nest, immediately snapping their toothed beaks and saying "hungry!" This means they are vicious and ravenous from the moment they hatch, though they seemed unable to do much with only their heads outside of their shells. After Jake spends a while getting his powers back, the birds are fully hatched and Gary is telling them to save Finn's brain for dessert. They walk towards Finn until Jake arrives and scares them off.
They are supposedly Gary's offspring. Jake saved Finn by scaring the Hatchlings away after knocking Gary back into the River of Junk. Their origins in relation to Gary are questionable due to the fact that they look nothing like their mother.
They are bird-like creatures that are slightly taller than Finn with light-colored beaks. They are mostly green and have yellow-green feathers with slanted red eyes, making them look menacing. They also have blue tongues and teeth in their beaks when they first snap at Finn.